1. Field
The present invention relates to apparatus for rapidly detecting the growth of a micro-biological organism within a selected nutrient media by measuring and recording a succession of changes in the electrical impedance of a small quantity thereof, and more particularly to such apparatus capable of digitally measuring and recording a succession of changes in the electrical impedance of a large number of samples differing from each other in either the organism or the nutrient media involved or both.
2. Prior Art
Heretofore, the usual procedure for identifying the presence and extent of a micro-biological organism, given a biological, medical, food, soil, sewage, etc., sample, has been to introduce the unknown micro-organism into a selected nutrient media to allow growth of the former. It is not unusual for days to elapse before the growth proceeds to the state where positive counting and identification of the microbial specimen can be performed. In fact, it is not unusual to culture a specimen for a week to several weeks in order to provide a positive identification and a quantitative analysis. Obviously, as in the case of medical diagnosis, such long delays are a decided handicap to a physician who accordingly may have to prescribe medical treatment for a patient without having early access to the positive identification of the micro-biological organism.
U.S. Pat. No. 3,743,581, issued to Paxton Cady and William J. Welch on July 3, 1973, teaches a novel method of and means for electrically measuring the growth of a micro-biological organism. The measurements are made by sampling the ratio of the voltages across two cells, one cell containing a nutrient media and being the "reference" cell, the other cell containing the same nutrient media innoculated with the subject micro-biological organism, and being the "specimen" cell. The cells define two legs of a current nulling bridge network of the Wheatstone type and the measurements are made by simple analog circuits.
Thus, the apparatus disclosed in U.S. Pat. No. 3,743,581 for practicing the novel method taught therein requires an operator to repeatedly balance the bridge circuit and read the measurements therefrom at different points in time for recording. In order to obtain useful results from such apparatus, it is necessary that the operator thereof possess a high degree of skill. Furthermore, any attempt to automate the operation of the apparatus is complicated by the relatively high sensitivity required of the apparatus for detection of the minute changes in the ratio of the voltages between the cells caused by growth of a micro-biological organism.
It is an object of this invention to provide apparatus for practicing the method taught in U.S. Pat. No. 3,743,581 capable of automatically producing a digital output representative of the ratio of voltages between a pair of cells at a given point in time with a high degree of accuracy.
It will be understood that efficient use of apparatus of this type requires that it be capable of measuring the voltage ratios of large numbers of cell pairs. in some cases all of the cell pairs will contain the same nutrient media with the specimen cell of each pair containing a different micro-biological organism, in other cases each of the cell pairs will contain a different nutrient media with the specimen cells of all of the cell pairs containing the same micro-biological organism and, finally, both the nutrient media and the micro-biological organism may be different in each cell pair of a plurality of cell pairs in certain applications. The wide differences in the amount of change produced by different micro-biological organisms in different nutrient mediae requires increased skill on the part of the operator of the prior art apparatus whereas the mere quantity of the readings required will tend to produce errors caused by fatigue and impose a maximum limit on the number of cell pairs that can be monitored.
It is another object of this invention to provide apparatus which is capable of automatically producing digital outputs each representative of the ratio of voltages between a different one of a large number of cell pairs with a high degree of accuracy and in a small amount of time and to do so repeatedly over an extended period of time.
In order to identify a particular micro-biological organism it is necessary to study the growth thereof in a large number of different nutrient media with and without specific antisera which inhibit or support growth of different micro-biological organisms. The resultant measurements of the voltage ratios between each of a large number of cell pairs made repeatedly over an extended period of time must be recorded in a manner which will enable them to be studied with respect to each other in order to arrive at an accurate identification. According to the prior art, the readings of the operator were recorded manually and subsequently manually processed with additional opportunity for error to produce a graphical presentation for study.
It is a further object of this invention to provide apparatus which is capable of automatically producing digital outputs each representative of the ratio of voltages between a different one of a large number of cell pairs which outputs can be conveniently recorded and compared to each other to provide early and accurate comparison therebetween.